A

Anywhere Fix

A receiver's ability to start position calculations, without being given an approximate time and approximate location.

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B

Bandwidth

The range of frequencies of a given signal.

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C

C/A Code or Course/Acquisition

The standard GPS Code. A sequence of 1023 binary, pseudo-random, bi-phase modulations on the GPS carrier at a chip rate of 1.023 MHz. AKA the "Civilian Code."

Carrier

The signal, that can be varied from a known reference, by modulation.

Carrier-Aided Tracking

The signal processing strategy, that uses the GPS carrier signal, to achieve an exact lock on the pseudo-random code.

Carrier Frequency

The frequency (of the unmodulated fundamental output) of a radio transmitter.

Carrier Phase GPS

The GPS measurements based on L1 or L2 carrier signals.

Channel

The GPS receiver's circuitry, necessary to receive any signal, from a specific, single GPS satellite.

Chip

An integrated circuit; The transition time for individual bits in the pseudo-random sequence.

Civilian Code

See: C/A Code or Course/Acquisition

Clock Bias

The difference between true Universal Time and the clock's indicated time.

Code phase GPS

The GPS measurements based on the C/A Code, opposed to the carrier of that code.

Control Segment

The world-wide network of GPS monitor and control stations, that ensure the accuracy of satellite positions and their internal clocks.

Cycle Slip

The discontinuity, in the measured carrier beat phase resulting from a temporary loss of lock, in the carrier tracking loop of a GPS receiver.

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D

Data Message

An embedded message in the GPS signal that reports the satellite's location, clock corrections and health. Included is rough information on the other satellites located in the constellation.

Differential Positioning

An accurate measurement of the relative positions of two receivers tracking the same GPS signal.

Dilution of Precision or Geometric Dilution of Precision

The multiplicative factor that modifies ranging error; Its sole cause is by the geometry between the user and his set of satellites. AKA DOP or GDOP

Dithering

An introduction of digital noise. The Department of Defense's process to add inaccuracy to GPS signals to induce Selective Availability.

Doppler-Aiding

A signal processing strategy, that uses a measured doppler shift, to help the receiver to track the GPS signal smoothly. Allows for a more precise position and velocity measurement.

Doppler Shift

The apparent change, in the frequency of a signal, caused by the relative motion of the receiver and transmitter.

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E

Ephemeris

The predictions of a satellite position, that are transmitted to the user, in the data message.

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F

Fast Switching Channel

A single channel that rapidly samples a number of satellite ranges. "Fast" implies that the switching time is sufficiently quick (usually in the range of 2-5 milliseconds) for recovering the data message.

Frequency Band

A particular range of frequencies.

Frequency Spectrum

Distribution of signal amplitudes as a function of frequency.

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G

Geometric Dilution of Precision or GDOP

See: Dilution of Precision

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H

Hardover Word

The word in the GPS message, that contains synchronization information for the transfer of tracking, from the Course/Acquisition Code to the Precise Code.

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I

Ionosphere

The band of charged particles, located approximately 80-120 miles, above the Earth's surface.

Ionospheric Refraction

The change in the signal's propagation speed it passes through the ionosphere.

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J

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K

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L

L-Bands or L1/L2 Bands

A group of radio frequencies that extend from 390 MHz to 1550 MHz. The GPS carrier frequencies (1227.6 MHz and 1575.42 MHz) are also in the L band.

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M

Multi-Path Error

Errors, caused by the interference of a signal, that have reached the receiver antenna by two or more different paths. Usually caused by one path being refracted or reflected.

Multi-Channel Receiver

A GPS receiver that can simultaneously track multiple satellites' signals.

Multi-Plexing Channel

A GPS receiver's channel, that can be sequenced through a number of satellite signals.

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N

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O

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P

P-Code or Precise Code

A very, long sequence of binary, pseudo-random, bi-phase modulations on the GPS carrier. The chip rate is at 10.23 MHz, which repeats about every 267 days. Each one week segment of this code is unique to one GPS satellite and is typically reset each week.

Precise Positioning Service (PPS)

The most accurate, dynamic positioning possible with standard GPS; it's based on the dual frequency Precise code and no Selective Availability.

Pseudo-Lite

A ground-based, differential GPS receiver that transmits a signal; similar to a GPS satellite.

Pseudo-Random Code

A signal with random, noise-like properties; a complicated, repeating pattern of ones ('1's) and zeroes ('0's).

Pseudo-Range

A distance measurement; it's based on the correlation of a satellite transmitted code and the local receiver's reference code, that has not been corrected for errors in synchronization (between the receiver's clock and the transmitter's clock).

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Q

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R

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S

Satellite Constellation

The arrangement of a set of satellites in outer space.

Selective Availability (SA)

A Department of Defense policy used to introduce some intentional clock noise into the GPS satellite signals, thereby intentionally degrading their accuracy for civilian users. This policy was discontinued as of May 1, 2000 and now SA is turned off (for now).

Slow Switching Channel

A sequencing, GPS receiver channel, that slowly switches, thereby preventing a continuous recovery of the data message.

Space Segment

Part of the entire GPS system that is in space; i.e., the satellites themselves.

Spread Spectrum

A system, in which the transmitted signal is spread over a frequency band much wider than the minimum bandwidth needed, to transmit the data being sent. This process is performed by modulating with a pseudo-random GPS code.

Standard Positioning Service (SPS)

The standard, civilian positioning accuracy obtained by using a single, frequency Course/Acquisition Code.

Static Positioning

A location's determination, when the receiver's antenna is presumed to be stationary on the Earth. This method allows the use of multiple, averaging techniques, that improve accuracy by factors of over 1000 percent.

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T

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U

User Interface

A display panel; The way a receiver conveys information (to the individual using the device); The controls or displays.

User Segment

Part of the entire GPS system, that includes the receivers of GPS signals.

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V

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W

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X

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Y

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Z

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